In general, this invention relates to drawn and ironed cans, and, in particular, to drawn and ironed can bodies having an improved end wall configuration.
The so-called drawn and ironed can has, to a large extent, replaced the old three-piece can in the beverage industry. These cans are made almost exclusively from aluminum, which, being quite ductile, is easily drawn into a cylindrical configuration and ironed down to a very thin wall thickness. While the economies of mass production are reflected in the low cost of the cans, the cost of the sheet aluminum stock from which the cans are manufactured has nevertheless always been an important consideration. Through the years various advances in can technology have enabled the can bodies to be manufactured from increasingly thinner aluminum sheet stock. This effort to manufacture drawn and ironed cans from increasingly thinner aluminum sheet stock is known as "light-weighting."
The typical drawn and ironed can consists of two components, namely a lid and a can body, only the latter of which is formed by the drawing and ironing process. When completed, the can body includes a very thin side wall and a domed end wall integrally formed with the side wall at one end. The opposite end of the side wall is joined to the lid along a seam after a beverage is introduced into the can body.
To form the can bodies, circular disks are first stamped or blanked from aluminum sheet stock of the appropriate thickness. Next, each disk is drawn into a cup. The cup is then placed over the end of a punch and forced through a series of dies where it is redrawn into a lesser diameter and ironed along its side wall to substantially reduce the thickness of the side wall while at the same time elongating it. The end wall, on the other hand, generally retains the original thickness of the sheet stock throughout the entire process. After the side wall is completely ironed, a domed configuration and surrounding rim is imparted to the flat end wall. This configuration enables the end wall to withstand predetermined internal pressures without buckling outward and rendering the can unstable. The pressure at which a given dome profile and thickness buckles outward is known as the dome reversal pressure or DRP. The domed configuration and surrounding rim also give the can adequate column strength.
The ability of a can to withstand internal pressure loads developed by the carbonated liquid therein during pasteurization, consumer storage, and consumer transportation is a result of, among other things, the bottom profile geometry and metal gauge of the can. The final can bottom profile is the result of two processes. The first process, which is performed after the last ironing operation, gives the can bottom its inward or convex bulge. Prior to the present invention, a punch assembly consisting of a punch sleeve and a punch nose was used to drive the flat base of the can against a metal dome plug having a configuration corresponding to that of the punch nose except that the diameter of the dome plug was less than the diameter of the domed region of the punch nose by a distance equal to twice the thickness of the metal of the can bottom plus 0.010" clearance. In other words, in the prior process, the clearance between the dome plug and the punch nose was minimized. Thus, in the prior process the diameter of the dome plug is identical to the diameter of the desired dome of the can end. The finished can after this stage of processing is called a "preform." Preform geometry is determined by the tooling installed in the body maker.
The second process is known as the base profile reforming or BPR process. In the BPR processes, the can bottom profile or geometry is reformed to increase its ability to withstand pressure loads, i.e., to increase its strength. To do this, the domed can bottom is driven against a BPR chuck having a diameter equal to that of the dome plug. However, unlike the dome plug, the BPR chuck has a horizontal upper surface, rather than a domed one. The finished can after the BPR process is termed a "reform." Reform geometry is controlled by the BPR process and its tooling geometry.
It has now been found that modifying the preform tooling by reducing the diameter of the dome plug so that the clearance between the dome plug and the punch nose is greater than the minimized clearance, and making the corresponding reduction in the diameter in the BPR chuck, the DRP of the finished can body can be increased or conversely, the can end can be light-weighted without a corresponding reduction in DRP. In particular, it has been found that reducing the diameter of the dome plug and making the corresponding reduction in the diameter of the BPR chuck by about 0.02" in relation to that which provides for a minimum clearance between the dome plug and the punch nose for a given can size, increases the dome reversal pressure by 2-3 psi. This allows an additional light-weighting of the can bottom by approximately 0.0002"-0.0003" beyond that which was capable with the existing process.